Water Vapor Transmission Rate (WVTR) Testing of Plastic Sheets: Methods, Standards, and Applications

Water vapor transmission rate (WVTR) testing measures how much moisture permeates through plastic sheets over time under controlled temperature and humidity conditions, making it a critical quality control parameter for packaging, pharmaceutical, and construction materials. This guide explains the testing principles, standard methods such as ASTM E96, ASTM F1249, ISO 15106, and GB/T 1037, sample preparation requirements, and practical applications of WVTR testing for plastic films and sheeting.

Technical Overview

Water vapor transmission rate (WVTR), also known as water vapor permeability, quantifies the steady-state flow of water vapor through a unit area of a plastic sheet per unit of time under specified temperature and humidity conditions. The result is typically expressed in grams per square meter per 24 hours (g/m²·24h). A lower WVTR value indicates a stronger moisture barrier and better protection of moisture-sensitive contents.

For plastic sheets and films, WVTR is one of the most important barrier performance indicators. It directly affects product shelf life, moisture protection of packaged goods, and the long-term reliability of materials used in construction, electronics, and medical applications. The permeation behavior of a plastic sheet is governed by several intrinsic and extrinsic factors, including polymer chemistry, crystallinity, molecular orientation, thickness, ambient temperature, and the relative humidity gradient applied across the specimen.

Because barrier performance can vary by orders of magnitude between materials—high-barrier foil laminates may transmit less than 0.1 g/m²·24h while ordinary polyethylene sheets may exceed 15 g/m²·24h under identical tropical test conditions—WVTR testing must be performed with appropriate methods, calibrated instruments, and strictly controlled environmental conditions in accordance with recognized international and national standards.

Testing Samples

WVTR testing is applicable to virtually all flexible and semi-rigid plastic sheet materials. Common sample categories submitted to accredited third-party laboratories and in-house quality control facilities include:

Single-Layer Polymer Sheets and Films

Typical materials include low-density polyethylene (LDPE), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), cast polypropylene (CPP), biaxially oriented polypropylene (BOPP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polystyrene (PS), polyamide (nylon, PA), cellophane, and polylactic acid (PLA) biodegradable sheets. Each polymer family exhibits a characteristic moisture barrier range, with polyolefins offering moderate barrier, PET providing good barrier, and hydrophilic materials such as nylon showing strong humidity dependence.

High-Barrier and Multilayer Composite Sheets

Multilayer co-extruded films, aluminum foil laminates, aluminum-metallized films, EVOH-containing structures, PVDC-coated films, and other functional composite sheets are tested to verify that barrier layers are intact and defect-free. These materials require high-sensitivity instrumental methods capable of detecting transmission rates below 0.5 g/m²·24h.

Rigid and Thermoforming Sheets

Thicker sheets used for thermoformed trays, blister packs, and rigid packaging—such as PP, PET, PVC, and PS sheets with thicknesses from 0.2 mm to 1 mm or more—are also evaluated, particularly when they serve as the primary moisture barrier for pharmaceuticals or dried foods.

Sample Preparation Requirements

• Specimens must be flat, crease-free, and free of pinholes, wrinkles, or visible defects within the test area.
• Specimens should be conditioned prior to testing, typically at 23°C and 50% relative humidity for a minimum of 4 hours, or as specified by the relevant standard.
• Specimen diameter is generally in the range of 50 mm to 140 mm depending on the test cell geometry, and thickness should be measured at multiple points and reported as the average.
• A minimum of three replicate specimens per material is recommended to account for material variability.

Testing Items

Core measurable parameters for a plastic sheet WVTR evaluation include:

• Water vapor transmission rate (WVTR), expressed in g/m²·24h under specified conditions.
• Water vapor permeance, the transmission rate normalized by the vapor pressure difference across the specimen, expressed in g/(m²·24h·kPa).
• Water vapor permeability coefficient, a material property corrected for thickness, expressed in g·cm/(cm²·s·Pa), which enables comparison between materials of different thicknesses.
• Barrier classification of the material based on measured results, commonly categorized as high barrier, medium barrier, or general purpose.

Testing is typically conducted under standardized climatic conditions, the most common being 38°C with 90% relative humidity (simulating tropical storage conditions), 23°C with 50% relative humidity (temperate conditions), and 40°C with 90% relative humidity (accelerated conditions for high-barrier materials). The selected condition must always be reported together with the result, since WVTR values are strongly dependent on the test environment.

Testing Methods

Two families of methods dominate plastic sheet WVTR testing: the traditional gravimetric cup method and modern instrumental sensor methods. Each is governed by specific international and national standards.

Gravimetric Cup Method (Desiccant Method and Water Method)

The cup method, standardized in ASTM E96/E96M, GB/T 1037, JIS Z 0208, and ISO 2528, seals the specimen over a cup containing either a desiccant (dry cup method) or distilled water (wet cup method). The assembled cup is placed in a controlled temperature and humidity cabinet, and the cup is weighed periodically. In the dry cup configuration the cup gains weight as vapor is absorbed by the desiccant; in the wet cup configuration the cup loses weight as vapor escapes through the specimen. WVTR is calculated from the linear slope of weight change versus time divided by the test area.

• Advantages: simple apparatus, low cost, broad applicability to thick and heterogeneous materials.
• Limitations: long test duration (typically several days to two weeks), limited sensitivity (practical detection limit around 0.5 g/m²·24h), and susceptibility to sealing and edge-leakage errors.

Infrared Sensor Method

The modulated infrared sensor method, specified in ASTM F1249, ISO 15106-2, and GB/T 26253, clamps the specimen between a humid chamber and a dry side swept by nitrogen carrier gas. Permeated water vapor is carried to an infrared sensor, and the steady-state signal, calibrated against reference films, yields the WVTR. The method offers rapid equilibration (often within a few hours), excellent repeatability, and sensitivity down to approximately 0.001 g/m²·24h, making it the preferred technique for high-barrier metallized and foil laminate sheets.

Electrolytic Sensor Method

Standardized in ISO 15106-3 and GB/T 21529, the electrolytic (coulometric) method directs the carrier gas carrying permeated water vapor through a cell coated with phosphorus pentoxide. The absorbed water is quantitatively electrolyzed, and according to Faraday's law the electrolysis current is directly proportional to the mass of water transported. This absolute measurement principle provides high accuracy and sensitivity suitable for medium- and high-barrier materials.

Humidity Sensor Method

Described in ISO 15106-1, the relative humidity sensor method monitors the humidity increase in a dry chamber separated from a saturated humid environment by the specimen. The rate of humidity rise is converted into a transmission rate through calibration. This method is well suited to automated multi-cell instruments and routine quality control.

General Test Procedure

• Condition and dimension the specimens, recording average thickness.
• Mount and hermetically seal each specimen in the test cell, ensuring no edge leakage.
• Establish the specified test conditions, for example 38°C and 90% relative humidity.
• Run the test until steady-state permeation is reached, typically defined as a variation of less than 5% between consecutive measurement intervals.
• Calculate WVTR, average the replicate results, and report the value together with test method, conditions, thickness, and specimen orientation.

Key factors influencing accuracy and reproducibility include temperature stability (permeation roughly doubles with each 8–10°C rise for many polymers), the humidity gradient, quality of sealing, absence of pinholes, complete conditioning of the specimen before testing, and instrument calibration using standard reference films traceable to certified values.

Applications

WVTR test data for plastic sheets support critical decisions across many industries:

• Food packaging: shelf-life modeling and material selection for snacks, baked goods, dried foods, powdered beverages, coffee, and condiments, where moisture gain or loss controls crispness, caking, and spoilage.
• Pharmaceutical packaging: qualification of blister films, sachets, and laminate structures for effervescent tablets, powders, and moisture-sensitive drugs, complementing stability studies.
• Medical device packaging: verification of sterile barrier systems and Tyvek-film laminates used for moisture-sensitive devices.
• Electronics: evaluation of moisture barrier films for OLED displays, photovoltaic backsheets, and packaging of moisture-sensitive components.
• Construction: classification of vapor barriers and vapor retarders for walls and roofing underlayments, where ASTM E96 results are used to assign Class I, II, or III retarder ratings.
• Agriculture and consumer goods: assessment of greenhouse films, silage films, cosmetic pouches, and protective sheeting.

FAQ

Which method should I choose for testing my plastic sheet: the cup method or an instrumental sensor method?

The gravimetric cup method is cost-effective and sufficient for general-purpose and medium-barrier materials where the expected WVTR exceeds roughly 1–2 g/m²·24h. For high-barrier materials such as metallized films, foil laminates, or EVOH co-extrusions, instrumental methods based on ASTM F1249 or ISO 15106 are strongly recommended because they offer far higher sensitivity and shorter test cycles. Because results from different methods may differ slightly, always compare data generated under the same standard and conditions.

What do the common test conditions of 38°C/90% RH mean?

These conditions simulate a tropical high-humidity environment and are the most widely used accelerated test climate for packaging materials. Alternative climates such as 23°C/50% RH represent ambient temperate conditions. Since WVTR increases sharply with temperature and humidity gradient, results from different climates are not directly comparable and must be clearly labeled with the conditions used.

How long does a WVTR test take?

Instrumental tests typically reach steady state within several hours, with total turnaround of one to three days including conditioning. Cup method tests generally require three to fourteen days depending on the barrier level of the material, because the specimen must reach steady-state permeation and enough weight change must accumulate for reliable measurement.

Can WVTR results be used to predict product shelf life?

Yes, when combined with additional data. By integrating the sheet WVTR, total package surface area, the moisture isotherm of the product, and the critical moisture content at which the product fails, packaging engineers can model moisture uptake over time and estimate shelf life. Testing at several temperatures also allows Arrhenius-type extrapolation of barrier performance.

Is WVTR the same as water absorption of the plastic?

No. Water absorption measures how much moisture a material takes into its bulk, whereas WVTR measures the rate at which moisture passes through the material. A hydrophilic film such as nylon may absorb significant water while still exhibiting a measurable transmission rate that changes with humidity, which is why WVTR must always be tested under controlled conditions relevant to the actual use environment.

How should specimens be taken from a roll of plastic sheet?

Specimens should be cut from representative locations across the web width, avoiding edges, wrinkles, and printed or damaged areas. For anisotropic materials such as oriented films, specimen orientation should be recorded, and for laminates the barrier layer side facing the humid chamber should be documented, since asymmetric structures can show direction-dependent permeation.